An archipelago of fractures formed along the rim of a 110-mile-wide (180-kilometer-wide) crater of a meteorite that hit the Yucatán 65 million years ago. Over time those fractures filled with water. Connected to the sea through caverns, the water rose and fell with ice ages. When dry, cenotes cracked and eroded, leaving soil packed with the bones of small mammals.

A telltale sign of the meteorite crater emerges in this gravity model: Readings are higher along the crater's perimeter, site of the ring cenotes. A 200-pound (90-kilogram) person standing on the rim would weid 199.998 pounds inside the crater.